1. Field of the Invention
The present invention relates to an inner bag for a transport tank, and more specifically, to an envelope type inner bag which is formed to fit the size of the transport tank.
2. Description of the Prior Arts
In cargo transportation by sea, railroad, road and so forth, a tank container is generally used for liquid materials (cargo). As the tank container, a 20 foot container (hereinafter referred to as a tank container) which conforms to the ISO Standards is ordinarily used, for example. The tank container has 20 foot length, 8 foot width, and 8 foot height, so that about 20 tons of liquid can be filled therein.
In the container transportation to use this kind of tank container, it is necessary to wash the inside of the tank after transportation, and in addition, to produce the tank by using a high quality stainless steel plate with chemical resistance. In order to solve the problems, Japanese Patent Laid-Open Publication No.S61-104983 discloses that an inner bag or liner bag made of soft synthetic resin to have the chemical resistance is loaded in the tank produced from the general steel plate. In addition, Japanese Patent Laid-Open Publication No. 2001-354292, Japanese Utility-Model Laid-Open Publication No.S61-48190, Japanese Patent Laid-Open Publication No.S50-4615, and Japanese Utility-Model Laid-Open Publication No.S57-46492 also disclose to load the inner bag in the tank in order to save the trouble for washing the inside of the tank.
However, with respect to the prior art inner bag to be used in the tanks and tank containers, it is so difficult to produce appropriate inner bags for large tanks including the 20 foot container that there has no practical application. Namely, it has been difficult to produce the inner bag fitting in the cylindrical 20 foot container easily and affordably. The ideal inner bag to fit within the tank container properly would be a cylindrical-shaped inner bag having approximately the same shape as the tank container. However, it is necessary to prepare circular lid films, and in addition, to weld the circular lid films on both ends of a tubular film. To make matters worse, since the circular lid film has to be welded not in a two-dimensional direction, but in a three-dimensional direction, the exclusive guide apparatus for welding the circular lid film is required.
In contrast, an envelope type inner bag is easily produced only by welding the both ends of the tubular film. This type of inner bag prevents the liquid from directly contacting with the inside of the tank by joining supply-discharge openings of the inner bag and the tank. Therefore, changing the inner bag makes it unnecessary to wash the inside of the tank. However, since the tank container is cylindrical, if the envelope type inner bag is loaded therein, the following problem occurs due to the difference in shape between them. The envelope type inner bag is sealed only by a welding line on each end. If the inner bag is not long enough, a gap is created between the tank container and the inner bag even if it is filled with liquid. An inertia force of the liquid in transporting concentrates as a load between the inner bag and the supply-discharge opening due to the gap. The inner bag may be torn at both the ends. Meanwhile, if the inner bag is unnecessarily long, it becomes impossible to adequately feed the liquid in folded portions at both ends of the inner bag because the folded portions are pressed down by the weight of the liquid already fed in, even before it is filled with the liquid. Accordingly, even if the inner bag has an enough capacity, a necessary amount of liquid cannot be filled therein. If the liquid is kept fed in the inner bag in a state where the folded portions are pressed down, the internal pressure of the inner bag becomes high to damage the inner bag. Although the envelope type inner bag is easily produced, if the inner bag is not formed to have the appropriate size for fitting in the tank container, the filling failure and the breakage of the inner bag may occur.